create cubemap from 6 images (with python)
You don't need to use Python to turn 6 cubic images into a single environment map. But once you set up a .blend for handling this task you could augment that with Python to make an automated process out of it.

Create a fairly large cube, with normals flipped, in your World center. UV map and texture your images onto it. Give it a shadeless material. Place your camera at location 0,0,0 with rotation X:90 Y:0 Z:0
Set your Camera to "Panoramic" to render an environment. I recently made a post here that goes into some more detail that you may find helpful. 
You'll want to make that cube spherical using a SubSurf and then Cast modifier. For the SubSurf modifier use "Simple" subdivision at 5 or 6 subdivisions. For the Cast modifier leave the type as "Sphere" and set the factor to 1.

If you want to use Python to process large quantities of such cube maps, look into how to use Python to replace the source images in the folder where Blender will be looking for them. And research how to use Python to call on Blender to load up the .blend and render out the image(s).